Several types of control strips are known. As described in U.S. Pat. No. 4,852,485, analogue control strips have been used, which consist of a piece of patterned film which could be attached onto a lithographic film before contacting it to a printing plate.
CH-A-0 681 929 describes a test "wedge" or control strip which is stored as a digital quantity on a storage medium such as a "floppy disk" or in a computer and is incorporated herein by reference. The control strip consists of a variety of control fields. Each control field contains a pattern, e.g. a star target, types, a line series, which may contain elements, e.g. checkerboard squares, lines or dots.
EP-A-0 518 559 describes a method and apparatus for creating a control strip. A digital representation of the control strip is printed to form a visible, analogue representation of the control strip. The control strip may be printed at the same time as a main coloured image to be reproduced. The control strip consists of control fields and the elements of the control fields can be user defined.
Postscript.TM. is a programming language created by Adobe Systems Inc., California, USA for defining page, lettering, colour and graphics parameters of images to be output by a raster imaging device such as a printer, an imagesetter or a platesetter. PostScript.TM. is described in the "PostScript Language Reference Manual", second edition, Addison-Wesley, 1990 (hereinafter referred to as "AdobeRef") and incorporated herein by reference. PostScript.TM. files may be incorporated in the file for a main image as an encapsulated PostScript file (EPS file), as described in Appendix H, pages 709-736 in AdobeRef.
The general techniques of colour reproduction, e.g. printing, photographic films, display devices, are described in "The Reproduction of Colour in Photography, Printing and Television", by Dr. W. G. Hunt, Fountain Press, UK, 1987 (referred to in the following as "KodakRef"). The Kodak "Three-Aim-Point" method is known from this book for producing printing plates that will yield results having consistent tone reproduction and colour balance despite Image Spot Size Deviation Variables (ISSDV) inherent in any system of image production or reproduction. One such variable or parameter is the development time for photographic film. So-called "hard dot" photographic films, such as Kodak Imageset 2000 IHN.TM. and imageset 2000 ILD.TM., are very sensitive to the development time period. If this period is short, then small spots are formed. If this period is longer, then larger spots or spots having a bigger area are formed. In several systems, such as in direct thermal systems, development is not required. In direct thermal systems, the size or the area of each spot may be influenced by the amount of energy applied to a specific microdot. Thus in such a system, one ISSDV is the amount of thermal energy locally applied.
Standard "originals" are provided in the form of three neutral density (grey) patches : original "A" represents a minimum reproducible density in an average transparency or reflection print, patch "B" is a similar maximum density patch, and patch "M" is a similar medium density patch. When the photographic material is processed, the patches are located next to the useful image and processed with it. As a result of experience, standard values and tolerances have been determined for the densities produced by the patches A, B and M on typical masks and separation negatives.
DE-A-19 507 665 discloses a control strip for visual control consisting of two adjacent longitudinal fields. The first field has large elements, the size of which is substantially independent from illumination variations. The target density of the large elements is position dependent. The second field has fine elements, having substantially the same tone value. The effective tone value depends strongly on the illumination conditions. Although it is possible to assess important illumination variations by this method, it is--due to the arrangement of the different fields--rather difficult to assess accurately small variations in process parameters and to get a quantitative measure for curing the effect.